The present invention relates to a method for thermally analyzing a material, comprising the steps of establishing a heat flow path between a sample of said material and a heat source to thereby cause a flow of heat between said sample and said heat source, controlling the heating power of said heat source as a function of time, measuring a signal representative of said heat flow between said sample and said heat source and a signal representative of a temperature associated with said heat flow, and evaluating a functional relation between said measured heat flow and temperature signals; and to an apparatus adapted for carrying out said method.
In the thermal analysis of materials, a sample of the material is heated by the heat source, and the flow of heat between the heat source and the sample is evaluated to thereby derive structural and compositional information about the material, in particular heat capacity, phase transitions, onset temperatures, etc. In particular, for the sake of accuracy and dynamic range, differential methods, e.g. differential scanning calorimetry (DSC), are being used. In these differential methods, a reference material is arranged in the heat flow symmetrically with respect to the sample to be analyzed, and the analysis is performed on the basis of the differential heat flow between the sample and reference materials.
EP 0 559 362 A1 discloses a differential method wherein the temperature of the heat source is controlled in accordance with a predetermined temperature program so as to cause said heat source temperature to vary in correspondence with a linear rise of temperature superposed by a periodic temperature modulation having a predetermined modulation amplitude and frequency. A deconvolution technique is used to derive from the differential heat flow signal two separate signal components caused by the linearly changing component and the modulation component of the heat source temperature, respectively.
WO95/33199 and WO95/33200 similarly disclose differential methods wherein a temperature of the heat source is driven through a predetermined temperature program, said temperature program comprising two linearly changing parts of the same time duration in the first case and a linearly changing part superposed with a periodically changing part having a predetermined amplitude and frequency in the second case. The differential heat flow signal and a phase difference between the differential heat flow signal and the programmed temperature of the heat source are evaluated to separately derive a real and an imaginary signal portion.
In these conventional methods, the thermal excitation of the sample is thus due to a linear rise in temperature combined with a temperature modulation of a selected amplitude and frequency. Since the resulting heat flow signal is dependent on the sample to be analyzed, and is therefore unknown, a problem arises how to select the temperature modulation. If the selected modulation amplitude is too small for the specific sample, the heat flow signal is too small, and the results are inaccurate. In contrast, if the selected temperature modulation amplitude is too big, the transfer of heat flow with the sample is too large thereby destroying the thermal event to be analyzed. This leads to time-consuming trial-and-error experiments until the appropriate temperature modulation amplitude is found.
It is an object of the present invention to provide for a method for thermally analyzing a material which is better adapted to heat flow requirements in each experiment and type of sample. It is a further object to provide for an apparatus which is capable of carrying out this method.
Having regard to the method, this object is attained in accordance with the invention in that said step of controlling said heating power is based on a first control input for causing said heat source to assume a predetermined temperature as a function of time and a second control input for modulating the heating power of said heat source caused by said first control input in accordance with a selected periodic power modulation.
According to the present invention, the user directly selects a modulation of the heating power of the heat source required for the experiment instead of selecting a temperature modulation for the heat source. This enables the user to directly determine the optimum heating power requirements for his individual experiment.
The terms xe2x80x9cheatingxe2x80x9d, xe2x80x9cheat flowxe2x80x9d, xe2x80x9cheat sourcexe2x80x9d and related terms are to be understood in the context of the present specification to mean either heating or cooling. In the latter case, the xe2x80x9cheat sourcexe2x80x9d will e.g. be a source of cooling agent thermally coupled to the sample.
The first control input for causing the heat source to assume a predetermined temperature as a function of time generally includes any temperature-versus-time function which varies with time considerably slower than the periodic power modulation caused by the second control input does. A particularly interesting specific case includes to vary the temperature of the heat source by the first control input in accordance with a linear temperature program which means a selected constant heating rate. Selecting the heating rate to be zero also includes the isothermic case where the temperature of the heat source is controlled to be constant at a selected temperature value by the first control input.
In one embodiment of the invention, the second control input is to set a predetermined amplitude of said power modulation. In this case, while both of the amplitude and frequency of the power modulation are fixed, the resulting temperature modulation of the heat source does no longer have a constant amplitude.
In another embodiment, the second control input is to determine said power modulation so as to result in a predetermined amplitude of said measured heat flow. This selects a constant amplitude of the measured heat flow while again the measured temperature amplitude is generally not constant.
It is useful to embody the invention so as to further comprise the steps of measuring a temperature of said heat source, filtering said measured temperature to thereby derive an average temperature corresponding to the unmodulated heating power of said heat source, and using a signal representative of a difference between said average temperature and said first control input as a heating power control signal for said heat source. The result is a first control loop which causes the average temperature to follow the unmodulated temperature-versus-time function commanded by the first control input.
When it is desired to control the amplitude of the resulting measured heat flow, it is useful for the method to further comprise the steps of demodulating said measured heat flow signal to thereby derive an amplitude of said heat flow caused by said power modulation, and using a signal representative of a difference between said demodulated amplitude and said second control input as a heating power control signal for said heat source. This corresponds to a second control loop causing the amplitude of the measured heat flow to assume an amplitude value commanded by the second control input.
The method according to the invention may be embodied so as to further comprise the steps of providing a supplementary heat source in addition to said heat source, controlling said heat source in accordance with said first control input, and controlling said supplementary heat source in accordance with said second control input. In this case, the heat source provides for the predetermined temperature-versus-time function while the supplementary heat source provides for the selected power modulation.
Preferably, the signal representative of heat flow is a differential signal corresponding to a difference of heat flows between said sample and said heat source and a reference material and said heat source. This provides for high accuracy and wide dynamic range since only the difference in heat flowing into or out of said sample as compared to the heat flowing into or out of a known reference material is used for the purposes of analysis, and there is no need for an absolute measurement.
In the case of differential analysis, the supplementary heat source may be associated with said sample only. This means that only the sample is exposed to the effect of the power modulation while the reference material is only subject to the effect of the temperature-versus-time function commanded by the first control input.
According to another important aspect, the method according to the present invention comprises the steps of deriving an average component of at least one of said measured heat flow and a heating rate derived from said measured temperature associated with said heat flow over a selected interval of time, deriving a dynamical component of at least one of said heat flow and heating rate as a difference between said measured heat flow or derived heating rate, respectively, and said respective derived average component, deriving an average temperature of said measured temperature associated with said heat flow over said selected interval of time, and representing at least one of said dynamical components as a function of said derived average temperature.
The dynamical component obtained by this type of evaluation is related to the power modulation of the heat source commanded by the second control input while the average component is related to the temperature-versus-time function commanded by the first control input.
While each of the heat source temperature or reference temperature could be used as a temperature associated with the heat flow, it is preferred that a temperature of said sample material is measured and is used as said signal representative of a temperature associated with said heat flow in the step of evaluating a functional relation between said measured heat flow and temperature signals.
The step for measuring the signal representative of the heat flow between the sample and the heat source and/or the reference and the heat source may advantageously be performed by measuring a temperature difference between at least two locations spaced at a distance along the respective heat flow path.
In order to perform the method in accordance with the invention, an apparatus for thermally analyzing a material comprising a heat source, a sample holder having a sample position thermally coupled to said heat source to thereby establish a heat flow path for a flow of heat between said heat source and a sample in said sample position, a controller for controlling the heating power of said heat source as a function of time, means for measuring a signal representative of said heat flow between said sample in said sample position and said heat source, means for measuring a signal representative of a temperature associated with said heat flow, and means for evaluating a functional relation between said measured heat flow and temperature signals is in accordance with the invention characterized in that said controller comprises means for setting a first control signal representing a selected temperature program of said heat source as a function of time and means for setting a second control signal representing a periodic power modulation of said heating power caused by said first control signal.
In the apparatus in accordance with the invention, the means for setting the second control signal may operate in various ways for selecting the parameters of the power modulation. It may e.g. be adapted to set a selected amplitude and a selected modulation frequency for the power modulation caused by the controller. Alternatively, it may be adapted to set a desired amplitude of the measured heat flow, and the controller in response thereto causes the power modulation to be performed so as to result in the set heat flow amplitude.
Specific embodiments of the apparatus in accordance with the invention are set out in subclaims 11 to 20.
In the following description, the method for thermally analyzing a material in accordance with the invention is exemplarily explained in conjunction with an apparatus adapted for performing the method with reference to the accompanying drawings, in which: